Wyo VB Lecture Notes

Objective #1: Use While loops.

• A While loop can be used to execute a block of code many times over and over. In a While loop, a control expression is typed after the keyword While. Visual Basic evaluates the control expression to either True or False. If the control expression is True then VB does iterate the loop. If it is False, VB skips the loop or stops iterating it. I prefer that you use parentheses around the control expression of a While loop for readability and style however VB does not require the use of parentheses.
  
  
• A While loop should be used instead of a For loop when you do not know the exact number of iterations that the loop must make. That is, if there is a situation in which some process must occur over and over again within a loop but you do not know when exactly it will end then you should use a While loop. For example, if you ask the user to input his test scores but your program does not necessarily know how many tests he has taken you could use a While loop and simply tell the user to enter the number -99 to end his list of input values. Of course, we know that -99 will not be confused with his actual exam scores since it is impossible to earn a -99 on an exam. In this case, -99 is called the sentinel value that warns the loop that the inputs have ended just like Paul Revere was a sentinel that warned the Americans that the British were coming.

• A While loop is called an indefinite loop (or indeterminate loop) because you, the programmer, usually do not know exactly how many times it will iterate. Remember that a For loop is a definite loop (or determinate loop) because a programmer can count how many times it will iterate based on its initial value, limit value, and step value.
  
  
• Here is an example of an accumulator statement in a While loop:                    

  intCounter = -1
  intTotal = 0
  intExam = 0
  
  While (intExam <> -99)
      intTotal = intTotal + intExam
      intCounter = intCounter + 1
      intExam = InputBox("Enter your next exam score (-99 to quit):")
  End While
  
  intExamAverage = intTotal / intCounter
  
  Note that in the example above the user is prompted by the InputBox to enter -99 to quit. Notice the use of an accumulator statement (intTotal = intTotal + intExam) as well as the use of a counter statement (intCounter = intCounter + 1). The loop allows the user to enter all of his or her exam scores. After the loop finishes iterating, the exam average is computed and stored in intExamAverage. Do you know why the variable intCounter is initialized to -1 instead of 0? Can you find the bug that would cause a potential run-time error in the example above? Can you rewrite the code to fix this logical error?

• The While loop in the example above is called a top-checking loop because the control expression which evaluates to true or false is placed at the top of the loop. It is also possible to use Do While loops as a bottom-checking loop with the control expression after the keyword Loop at the bottom of the loop. A bottom-checking loop guarantees at least one iteration while a top-checking does not since the control expression could evaluate to false right away. Here is an example of a bottom-checking Do While loop:
  
  Do
      intTotal = intTotal + intExam
      intCounter = intCounter + 1
      intExam = InputBox("Enter your next exam score (-99 to quit):")
  Loop While (intExam <> -99)
  
  
• You can use an Exit While statement to exit or "break out of" a While loop. You can also use an Exit For statement to break out of a For loop. However, it is not wise to use these Exit statements unless there is no other practical way to make the loop end naturally under the control of its control expression.
  
  Example:
  
  While (intExam <> -99)
      intExam = InputBox("Enter your next exam score (-99 to quit): ")
     
      If (intExam = -99)
          Exit While
      End If
  
      intCounter = intCounter + 1
      intTotal = intTotal + intExam
  End While
  
  in this loop, the loop immediately stops iterating if the user enters an exam score of -99 before the intCounter counter variable is incremented by one.
  
  
• Be careful not to create infinite loops that iterate forever. If a While loop's control expression never becomes False, the loop will never end possibly causing the user to be forced to turn off the computer itself! If a While loop is constructed like this:
  
  While (1 = 1)
  
  it may never finish iterating.
  
  
• A flag variable is often used to store the value 1 (meaning True) or 0 (meaning False). The programmer uses the flag variable as a way of controlling some process in an algorithm. Flag variables are often used in loops to send a signal to the control expression. For example, in the code segment below intGameIsOver is used as a flag variable. It sends a signal to the control expression of the loop if the user has scored enough points to end the game and exit the loop. intGameIsOver is initially set to the value 0 which indicates False (the game is NOT over). You should name flag variables appropriately. The variable should be named in a way that it's name indicates a True condition. (For example, in the segment below, you should not use a flag variable with the name, intGameIsNotOver, since it would probably make the logic of the algorithm harder for most fellow programmers to follow.)
  
  intGameIsOver = 0
  
  While (intGameIsOver = 0)
             ' a bunch of statements would be located here
             intUserPointsScored = intUserPointsScored + intLastRoundScore
  
             If (UserPointsScored > 1000) Then
                    intGameIsOver = 1
             End If
  End While
  
  
  Often programmers use variables with the Boolean data type as flag variables. A Boolean variable can be set to either True or False. Here is an example:
  
  Dim blnGameIsOver As Boolean = False
  
  While (blnGameIsOver = False)
             ' a bunch of statements would be located here
             intUserPointsScored = intUserPointsScored + intLastRoundScore
  
             If (intUserPointsScored > 1000) Then
                    blnGameIsOver = True
             End If
  End While
  
  By using a Boolean variable you can simplify the first line of the loop like this
  
  While (Not blnGameIsOver)
  
  Here's another example of a While loop that uses a flag variable. In this case, the flag variable has the Boolean data type.
  
  Dim blnOutOfBounds As Boolean = False
  
  While (blnOutOfBounds = False)
     picPlayer.Left = picPlayer.Left + 10
  
     If (picPlayer.Left > 300) Then
        blnOutOfBounds = True
     End If
  
  End While
  
  
• You should always surround your loops with blank lines in your code. This makes your program easier to read and debug. That is, above all While and For loops you should include a blank line and below all End While and Next statements, you should include a blank line. In fact, your instructor may deduct points for the failure to do so even though Visual Basic does not require these blank lines. Blank lines used to make code easier to read are called white space. Tabs and blank spaces that are also used for the same purpose of making white space.